I have been experimenting with seasonal changes for grass. Two aspects specifically :
* color gradually changes from light green (spring) to a dark green (summer) and finally to brown (fall). This is more artistic than realistic but i think it looks nice in a game like daggerfall.
* grass size changes a bit to smooth transitions to and from winter. Height is highest on summer and decrease simmetrically towards winter on both spring and fall (like this).

This is what i have right now

Spoiler!

Would you be interested in something like this? Optimally trees should have a similar behaviour with growing/falling leaves etc. but i think even grass alone has a considerable effect on the general mood.

If you are interested in creating mods for Daggerfall Unity you can find the documentation here.